JPS59107023A - Manufacture of hyperfine-grained hot-rolled steel plate - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel plate balanced between strength and ductility and having a hyperfine ferrite crystal structure by subjecting a steel contg. a prescribed percentage each of C, Si, Mn and other alloy elements to final hot rolling once or twice at a specified total draft in a specified temp. range within a fixed time. CONSTITUTION:A steel contg. <=0.3% C, <=1.5% Si, <=2.0% Mn and <=3% other alloy elements is prepd. The steel is hot rolled. In the final hot rolling stage, the steel is rolled once or twice or more at >=65% total draft in the temp. range of 600 deg.C- the Ar3 point within 2sec. In this method the ferrite structure is sufficiently processed, so it is recrystallized during or immediately after the hot rolling. The resulting structure is a hyperfine-grained structure, the shape of the grains is not remarkably elongated, and the hot rolled steel plate shows superior properties as an almost isotropic high tension steel plate balanced between strength and ductility.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a fine-grained steel sheet having an extremely fine ferrite crystal structure as hot-rolled and having an excellent strength-ductility balance.

The fine grain structure referred to here is composed of a fine ferrite phase9, depending on the desired mechanical properties? 'i7 In addition to the elite phase, it may contain one or more other fine structures, such as thuyarite, bainite, martensite, retained austenite, etc., or it may contain precipitates such as carbon and nitrides. You may have one.

The structure referred to as fine-grain ferrite in the present invention is almost isotropic without significant elongation of the grain shape, and as a general rule, it means a structure consisting of crystal grains surrounded by so-called high-angle grain boundaries. It is not considered as a subgrain boundary.

Conventionally, methods for strengthening steel materials include solid solution strengthening and hard microstructure (
martensite, bainite, etc.), precipitation hardening, and dislocation strengthening (work hardening, etc.).

Refining of crystal grains is known. Among these strengthening methods, refining the crystal grains is the only method to increase the strength and toughness and to improve the strength-ductility balance of the thin plate. Moreover, since it is not a strengthening method using ingredients, the ingredient cost can be kept low, so if unprecedented fine grains can be obtained, high-strength steel sheets of high quality and low cost can be manufactured.

In the conventional technology, the smallest ferrite grain size of 3 to 4 μm has been obtained in actual production, but it has been extremely difficult to obtain a grain size smaller than this.

By the way, the following methods have been conventionally considered for reducing the size of ferrite grains after transformation.

(1) Refinement of austenite grain size before transformation.

During austenite/ferrite transformation, da-phase nuclei are mainly generated on austenite grain boundaries. Therefore, making the austenite grain size finer and increasing the grain boundary area increases the density of transformation nuclei and reduces the ferrite grain size after transformation.

In order to reduce the austenite grain size, it is generally performed to repeatedly perform annealing recrystallization in the recrystallization temperature range. If static recrystallization occurs at this time, the grain size will be inversely proportional to the rolling reduction.
If dynamic recrystallization occurs, the grain size becomes smaller as the strain rate increases and the temperature decreases.

(2) '&I allow strain to remain in the austenite before I.

When austenite is processed in the non-recrystallized temperature range, a portion of the processed material is not released and accumulates, forming areas of high strain called deformation zones within the grains, which serve as nucleation sites for ferrite transformation. If the processing amount is sufficiently large and the deformation band density is small, the ferrite grain size after transformation will be small. This method is a technique called controlled rolling, and the smallest ferrite grains can be obtained in the conventional technique, and it is possible to obtain crystals of 3 to 4 μm industrially.

(3) Increase the cooling rate during transformation.

When the cooling rate is increased during austenite/7-erite transformation, the degree of supercooling increases, resulting in a greater number of transformation nuclei being generated, and since the temperature is lowered more quickly, grain growth is also suppressed and the ferrite grain size becomes smaller.

Although the methods (1) to (3) above are well-known grain refining methods, there is a limit to the ferrite grain size obtained by these methods. In other words, in the conventional method (1) of refining austenite, the maximum austenite grain size that can be achieved in commercial production is about 5μ, whether it is static recrystallization or dynamic recrystallization. Even if this is done, the austenite grain/ferrite grain conversion ratio approaches 1 as the austenite grain size becomes smaller, so the resulting ferrite grain size cannot be close to the austenite grain size before transformation.

In conventional method (2), the so-called controlled rolling method that introduces deformation zones into unrecrystallized austenite, fine-grained ferrite can be obtained as long as the rolling reduction in the unrecrystallized region is sufficiently large, but in actual production, The limit is about 3μ. Moreover, in this method, alloying elements such as Nb must be added in order to widen the non-recrystallization temperature range, and the addition of alloying elements inevitably results in higher costs.

In the conventional method (3), the higher the cooling rate, the better, but if it is too high, ferrite transformation will be suppressed and a quenched structure such as martensite will be generated, so there is a limit naturally.

As mentioned above, the conventional grain refining method has its limits, and in actual production it is very difficult to obtain ferrite grains of 3 μm or less, and the manufacturing cost is high.

As a result of researching the composition system and rolling/cooling process of hot-rolled steel sheets, the present inventors discovered a principle completely different from the conventional grain refining method described above. They developed a new method for manufacturing high-strength hot-rolled steel sheets consisting of a microstructure.

That is, the gist of the present invention is as follows.

(1) c 0.3% or less, Sil, 5% or less, Mn
When hot rolling steel having an i content of 2.0% or less and other alloying element content of 3% or less, in the final stage of the hot rolling,
A method for producing an ultra-fine grain hot-rolled steel sheet, characterized in that processing is performed once or twice or more within 2 seconds in a temperature range of 600° C. to Ars so that the total rolling reduction becomes 65 inches or more.

(2) C0.3% or less, S11.5% or less, Mn
When hot rolling steel with a content of 2.0% or less and a content of other alloying elements of 3 or less, at the final stage of the hot rolling stage, it is rolled once within 2 seconds in the temperature range of 600°C to Ar. or 2
Ultra-fine grain hot rolling that is characterized by being able to reach a temperature of 600°C or less at a cooling rate of 10°C/see or more within 5 seconds after being processed so that the total rolling reduction ratio is 65% or more. Method of manufacturing steel plates.

The present invention will be explained in detail below.

The ultra-fine grain structure of the steel according to the present invention is obtained by dynamically or unidirectionally recrystallizing the ferrite structure through processing such as rolling.

It is known that under conventional hot rolling conditions, recovery of the ferrite structure progresses significantly due to high stacking fault energy, and recrystallization is difficult in such a hot rolling process. but,
According to the present invention, the darkened ferrite structure is also sufficiently processed, so that it recrystallizes during or immediately after hot rolling.

The structure thus obtained is an ultra-fine grain structure without significant elongation of the grain shape, and exhibits excellent properties as a high-strength hot-rolled steel sheet that is nearly isotropic and has a good strength-ductility balance.

However, since the above-mentioned structure is fine-grained, once grain growth begins, the grain growth rate is fast.Therefore, if the grain growth is not rapidly cooled within a short period of time after hot rolling, some materials may exhibit significant grain growth depending on the components, so it is difficult to finish hot rolling. It must be rapidly cooled down immediately afterwards.

The reasons for the limitations of the present invention will be described below.

The reason for limiting the chemical composition of the starting steel of the present invention is as follows.

First of all, the reason why the carbon content is set to 0.3% or less is because generally, as the carbon content increases, the ferrite content decreases, and the amount of ferrite, etc. increases, making it difficult to obtain ferrite-based steel. be.

31 not only increases the strength but also has the effect of significantly raising the γ→α transformation starting temperature, and is added to broaden the temperature range between the A r s transformation point and the temperature at which ferrite recrystallization is possible, which is the processing condition for the steel of the present invention. Although desirable, if it exceeds 1.5%, the deterioration of ductility will be significant, so it is set to 1.5% or less.

Addition of Mn strengthens the steel in the same way as Sl, but addition of more than 2.0 mm is inappropriate in terms of melting or cost.

The reason why the total amount of alloying elements other than those listed above is limited to 3% or less is that if more than this is added, the Ars transformation point may become too low or fine precipitates may be formed, resulting in insufficient recrystallization of the ferrite structure. First, it shows an elongated processed structure and shows a large deterioration in ductility.

Next, rolling conditions in the present invention will be explained.

The rolling conditions specified in the present invention, i.e., 600% at the final stage.
The rolling conditions in which the total rolling reduction of 65 mm or more is performed once or twice within 2 seconds in the temperature range of °C to A r 3 are suitable conditions for recrystallizing the ferrite structure during or immediately after processing. .

If the rolling reduction is less than 65, ferrite will not sufficiently recrystallize in the hot rolling process. The same thing can be said when the processing temperature drops below 600°C. On the other hand, the processing temperature is Ars
When the transformation temperature is exceeded, the ferrite grains become finer because the austenite grains become finer and the sifunirite formation site tf increases, which is different from the intention of the present invention. For dynamic recrystallization of ferrite e, rolling in a two-phase region in the temperature range of Ars to Ar1, in which austenite is partially contained, is particularly advantageous.

The hot-rolled steel sheet obtained according to the limiting conditions of the present invention described above has a very fine structure, but in some cases, if it is not cooled immediately after hot rolling, grain growth will occur, making it difficult to obtain a fine-grain structure. . Therefore, within 5 seconds after hot rolling, 10㎜B
At the above cooling rate, it is necessary to cool to a temperature of 600° C. or less, and when it is desired to obtain a two-phase structure with martensite, it is necessary to cool to a temperature of 250° C. or less. On the other hand, in the case of thin hot-rolled steel sheets, the above cooling rate can be obtained even by air cooling, and it is possible to obtain a sufficiently fine structure.

Next, description will be made based on all embodiments of the present invention.

Steel having the chemical composition shown in Table 1 was rolled on the rolling schedule shown in Table 2. Table 3 shows the results.

In this example, the slab heating temperature was 1250° C., but low-temperature heating is advantageous both in terms of thermal energy and in terms of promoting grain refinement.

From the results shown in Table 3, implementation numbers 1, 2, and
The steel plates obtained in Examples 6, 7.8, and 12 all exhibit ultrafine grain structures and exhibit materials with an excellent balance of strength and ductility. On the other hand, steel sheets obtained by rolling and cooling outside the scope of the present invention exhibit a deformed structure, do not have a fine grain structure and have extremely poor ductility (Example No. 3 and 5), or cannot achieve sufficient strength. First, it is clear that a high-strength hot-rolled steel sheet with an excellent strength-ductility balance as intended by the present invention cannot be obtained.

In addition, as this example shows, in the case of material B, a strength of 70 kg/1 nm class can be obtained when rolled at 550 degrees Celsius, and a strength of 80 kg/lnm'' class can be obtained when rolled at 200 degrees Celsius or lower. Even low carbon steel, material A, can achieve a strength of 50 ky/Tomo 2 grade, making it possible to produce hot-rolled steel sheets with a wide range of strengths using a single component using conventional manufacturing methods and the manufacturing method of the present invention. became.

Claims (2)

[Claims] (1) C0.3% or less, Si1.5% or less, Mn
2.096 or less and the other alloying element content is 3 or less, in the final stage of the hot rolling, the steel is rolled once or twice within 2 seconds in the temperature range of 600°C to Ar3.
A method for producing an ultra-fine-grained hot rolled steel sheet, characterized by adding processing such that the total rolling reduction of 6,596 times or more becomes 6,596 times or more. (2) C0.3% or less, Si 1.5% or less, Mn 2.
0% or less, and the content of other alloying elements is 3 T or less, in the final stage of the hot rolling, 6.
After applying processing such that the total rolling reduction rate is C5 or more within 2 seconds in the temperature range of 00℃ to Ars, the cooling rate is 10℃/or more within 5 seconds. 600℃
A method for producing an ultra-fine grain hot-rolled steel sheet, characterized by bringing the temperature to the following temperature.